Center a movie clip in the Blender VSE
Just do some math in the X offset field directly. Look at the Original Dimension of your Clip where you will find out its width x height. Then type into the X offset "1920/2-width/2" (replacing width with the actual one from its Dimensions) and hit Enter. Do the same with Y, typing "1080/2-height/2"...

Or with an Editor/View of the type Python Console opened and the Clip selected in the VSE you should be able to just run this line of code to center your Clip/ImageSequence:

seq = bpy.context.selected_editable_sequences[0]; seq.use_translation = True; seq.transform.offset_x = bpy.context.scene.render.resolution_x/2 - seq.strip_elem_from_frame(bpy.context.scene.frame_current).orig_width/2; seq.transform.offset_y = bpy.context.scene.render.resolution_y/2 - seq.strip_elem_from_frame(bpy.context.scene.frame_current).orig_height/2